The specific aim of this research is to analyze in detail the cellular architecture of the thalamus and brainstem of the rhesus monkey (Macaca mulatta) for the purpose of producing two cytoarchitectonic atlases to be used as guides for experimental neuroanatomical and neurophysiological investigations of the organization and function of the macaque brain. Each atlas will be illustrated with three series of photomicrographs of cell-stained sections of celloidin-embedded brains, cut in standard frontal, sagittal and horizontal stereotaxic planes respectively. Cellular aggregates will be identified by labels on the illustrations, and stereotaxic coordinates will be indicated by a coordinate grid around each photomicrographic plate. Comparative stereotaxic information for the related species Macaca fascicularis will also be provided. In each atlas a text accompanying the illustration will provide a cytoarchitectonic and topographical description of each nucleus. In addition, the text will include a historical review of previous descriptive studies and alternative terminologies relating to each cellular entity, and a review of the significant literature on experimental investigations of its connections and, if known, its functional capacity. Finally a list of references and a detailed index will be provided. As a companion volume on the hypothalamus of the rhesus monkey is already in preparation by another investigator in our group, completion of the project proposed here will make available a complete experimental guide to the macaque brain (with the exception of the cerebral and cerebellar cortices). As study of the subhuman primate brain is of great importance to the investigation of the human brain, this project should be relevant not only to basic neuroscience but also to clinical neurology.